It has been suggested that the requirement for bandwidth in the public telecommunications network doubles about every two years ("Gigabit Networking on the Public Transmission Network", James A. McEachern, IEEE Communications Magazine, Vol. 30, No. 4, April 1992, pp. 70-78). The Synchronous Optical Network (SONET) OC-48 System (2.488 Gb/s) has been in service since 1992 which would imply that a bandwidth of 10 GB/s will be required by 1996 to meet demands. In the aforementioned article by James A. McEachern, it is indicated that in spite of current transmission rates in excess of 2 Gb/s less than 0.1% of the potential capacity of a single mode optical fiber is being exploited. The potential optical bandwidth available on single-mode fiber in the 1200 to 1700 nm low loss window is reported to be as much as 60 Tb/s.
Although bandwidths of up to 10 Gb/s have been demonstrated the practical upper limit of such a system has not yet been determined. Optical transmitters utilizing modulated lasers and optical modulators are capable of bandwidths to 10 Gb/s and beyond but other system elements such as optical detectors and electronic components are seen as potential limiting factors.
Since single-mode fiber has already been widely installed it follows from an economic perspective that it is advantageous to more fully utilize existing networks rather than routing additional fiber links. One method of more fully utilizing existing fiber while operating within the potential bandwidth limitation of 10 Gb/s is to deploy wavelength division multiplexing (WDM). By this process the bandwidth range within the low-loss window of 1200 to 1700 nm is partitioned into a plurality of discrete wavelength channels each potentially carrying a 10 Gb/s bandwidth. Each modulated wavelength carrier is multiplexed at the transmitter end and the composite signal is caused to propagate through the single-mode fiber. At the receiver end the composite signal is demultiplexed into the individual wavelength carriers and transferred to appropriate detectors.
A WDM access network may be established in at least two ways as shown in FIGS. 1A and 1B. According to the network of FIG. 1A, tunable lasers are employed to generate several discrete wavelength channels. Each wavelength channel is modulated by the external modulator and the individual, modulated channels are multiplexed into the fiber via the star coupler. At the receiver end the individual wavelength carriers are separated by preselected narrow band filters and read by photodetectors. The second network shown in FIG. 1B relies on fixed wavelength lasers to generate the wavelength carriers. These carriers are modulated by the external modulators and multiplexed into the fiber. At the receiver end a tunable filter is used to separate individual wavelength carriers.
Because optical fibers have a random polarization property the central carrier wavelength at the receiver end will include both transverse electric TE and transverse magnetic TM polarization modes. Generally, the wavelength selective coupling of the TE polarized mode in a directional coupler wavelength filter occurs at a longer wavelength than does coupling of the TM polarized mode. The difference in wavelength between the two modes can be 30 nm or more and hence an optical filter with a narrow bandwidth is unable to process both modes.